Rename unexported funcs, vars to match common Go

[alexandear]

This PR makes stylistic changes in internal code. It renames unexported functions and variables to match the common Go style:

• cancel_ctx -> cancelCtx
• svrUrl -> srvUrl (should be srv from server, not svr)
• isHttps -> isHTTPS
• isHttp -> isHTTP
• isIp6 -> isIP6
• isIp6WithoutPort -> isIP6WithoutPort

See https://github.com/golang/go/wiki/CodeReviewComments#initialisms and https://go.dev/doc/effective_go#mixed-caps.

[erikdubbelboer]

Thanks!